The present disclosure relates to a fixing device including a separation member for separating a sheet that has stuck to a rotator such as a fixing rotator, and to an image forming apparatus including the fixing device.
An image forming apparatus such as a printer, a facsimile or a copier includes a fixing device that applies heat and pressure to a sheet with toner thereon so that the toner is fixed to the sheet. The fixing device includes a fixing rotator and a pressure rotator, wherein the fixing rotator transmits the heat to the sheet, and the pressure rotator is made pressure-contact with the fixing rotator, and a nip portion is formed when the rotators make pressure-contact with each other. When the sheet passes through the nip portion, a toner image carried on the sheet is fused and fixed to the sheet. The sheet to which the toner image has been fixed is conveyed from the nip portion to the discharge tray. At this time, the sheet may be stuck to the surface of the fixing rotator due to the adhesive force of the fused toner, resulting in a conveyance error and occurrence of a paper jam. As a result, a separation member may be disposed on more downstream side than the nip portion in the rotation direction of the fixing rotator such that the sheet is separated from the fixing rotator by the separation member.
The separation member includes a contact type and a non-contact type, wherein the contact-type separation member causes a separation claw to be in contact with the surface of the fixing rotator, and the non-contact-type separation member separates the sheet by using a thin metal separation plate disposed at a  position distanced away from the surface of the fixing rotator. Color image forming apparatuses are more easily influenced than monochrome image forming apparatuses by a flaw generated on the surface of the fixing rotator and a potential difference generated on the surface of the fixing rotator. As a result, many color image forming apparatuses include a separation plate which is a non-contact-type member. In that case, it is necessary to maintain, with a high accuracy, a gap between the surface of the fixing rotator and a tip of the separation plate that is close to the fixing rotator.
In general, a color toner image is composed of overlaid toner images of different colors, and thus has a larger amount of toner carried on the sheet than a monochrome toner image. As a result, in color image forming apparatuses, a sheet is likely to stick to the surface of the fixing rotator by the fused toner. To prevent the sheet from sticking, the surface of the fixing rotator is often made of a fluorine material. For example, in the case where the fixing rotator is a fixing roller, the surface of the fixing roller is coated with a layer of pure fluororesin. In the case where the fixing rotator is a fixing belt, a PFA tube (fluororesin tube) is used for the surface of the belt.
In the fixing device as such, when a plurality of sheets having a relatively strong frictional force pass through the nip portion continuously in a low-temperature low-humidity environment, the surface of the fixing rotator is frictionally charged. In the charging series, fluorine is located on the most minus side, wherein the charging series indicate levels of charging that occur when different substances are frictioned. As a result, by the frictional charging, the surface of the fixing rotator is charged in minus. As the number of sheets continuously passing through the nip portion increases, the amount of charging on the surface of the fixing rotator increases. With the increase of the amount of charging, a local discharge may occur between the tip of the separation plate and the surface of the fixing rotator. In that case, the surface of the fixing rotator will have a part from which electricity has been removed by the local discharge, and a  part where the charged state is maintained. When the part from which electricity has been removed approaches the nip portion as the fixing rotator rotates, a phenomenon called electrostatic scattering occurs in which toner that has not been fixed to the sheet is scattered.
As the countermeasure for the electrostatic scattering, there are known, for example, a method of forming the surface of the fixing roller from a material other than the pure fluorine, a method of removing electric charges from the separation plate, and a method of making the separation plate electrically floating. For example, as the method of forming the surface of the fixing roller from a material other than the pure fluorine, the fixing rotator is formed from an electrically conductive PFA tube mixed with an electrically conductive material such as carbon. This restricts electric charges from being charged on the surface of the fixing rotator. However, this method introduces another problem where toner and paper dust stick to the surface of the fixing rotator. This is because the electrically conductive PFA tube has less sheet separation performance than pure-fluorine-based PFA tube. As the method of removing electric charges from the separation plate, for example, a separation plate having a polished end portion may be used. In this method, a local discharge is generated frequently onto the separation plate from the surface of the fixing rotator so as to restrict the amount of charging on the surface of the fixing rotator. However, since it is difficult to control the location and the frequency of the local discharge occurrence, the method could not be the fundamental solution to the electrostatic scattering. As the method of making the separation plate electrically floating, for example, a discharge path in which electric charges flow may be interrupted. This makes it possible to restrict the local discharge from being generated, and restrict the electrostatic scattering. However, with the increase of the amount of charging on the surface of the fixing rotator and the rise of the potential, problems such as electromagnetic wave noise in the image forming apparatus and abnormality in the image fixed on the sheet may occur. In addition, the charging of the separation plate itself may cause the sheet to slide on  the separation plate when being conveyed, leading to problems such as an image quality degradation and even a paper jam.
As a conventional technology dealing with the above-mentioned problem, there is known an image forming apparatus that restricts the electrostatic scattering by switching the fixing rotator and the pressure rotator between the grounding state and the floating state.